1. Field of the Invention
The present invention relates to a fuel injection control apparatus that is used in diesel engines and the like and which controls a fuel injection rate by adjusting an injection valve opening pressure of an injection nozzle.
2. Description of the Related Art
In fuel injection control, in addition to controlling the injection start time and the injection quantity, it is necessary to control the injection rate in order to achieve a desired injection waveform. Among control apparatuses of this type in the prior art, Japanese Unexamined Patent Publication S61-96163, for example, discloses an apparatus that controls the injection valve opening pressure of the injection nozzles with hydraulic pressure, achieved in this particular case by a technology through which deviation of injection rate characteristics and changes over time are eliminated by feed-back control of the oil pressure in such a manner that the peak value of the injection valve opening pressure matches a target value.
In a typical pump line nozzle system (a system which supplies fuel that has been compressed by a fuel injection pump to a nozzle via piping) or a typical unit injector, the injection quantity output from the time when the fuel is first injected from the nozzle until it is ignited (the period preceding the start of the main injection) is considered to be an important factor in indicating the characteristics of the fuel system. Alan P. Gill, in "Design Choices for 1990's Low-Emission Diesel Engines," SEA Technical Paper Series, 880350, notes that it has been verified that the smaller the injection quantity during this initial injection rate control period, the smaller the discharge of NOx. Consequently, injection control through the entire injection period, and in particular, during the initial injection rate control period, must be performed in accordance with various operating conditions such as the engine load, the rpm and the like.
However, the prior art described above performs control only to the extent that the peak value of the injection valve opening pressure is matched to a target value and it is not, therefore, possible to control the injection characteristics during the initial injection rate control period with great precision. In order to overcome this problem, control processing based upon a new concept is required.